1. Field of the Invention
The present invention is directed to a perpendicular magnetic recording medium. In particular, the present invention is directed to a perpendicular magnetic recording medium that includes a soft magnetic underlayer that incorporates exchange coupling induced anisotropy for improved magnetic recording properties, including an improved signal-to-noise ratio (SNR) and reduced adjacent track erasure.
2. Description of Related Art
Thin film magnetic recording media are composed of multiple layers, including one or more magnetic recording layers, disposed on a substrate. Typically, the magnetic recording layer includes small magnetic grains that have an easy magnetization axis that is magnetically oriented longitudinally (i.e., in plane) with respect to the magnetic layer.
The areal density of longitudinal magnetic recording media has been increasing at a compounded growth rate of about 60% per year and areal densities as high as 100 gigabits per square inch (Gbit/in2) have been demonstrated. Scaling longitudinal recording media to higher areal densities requires smaller magnetic grains. However, as the grain size is reduced, thermal fluctuations can cause the magnetic domains to “flip”, causing a loss of magnetization over a period of time. Media having a higher magnetic coercivity and an increased track density (tracks per inch, or TPI) can mitigate this problem. However, the large write head gaps that are needed for good overwrite of high coercivity media lead to excessive fringing, negatively affecting the data written on adjacent tracks.
Perpendicular (vertical) magnetic recording media have been proposed as a way to increase areal densities beyond 100 Gb/in2. Perpendicular magnetic recording media include a magnetic recording layer having an easy magnetization axis that is substantially perpendicular to the magnetic layer. A perpendicular write head, such as a monopole write head or a shielded pole write head, is utilized to magnetize the grains in the perpendicular recording layer. Examples of perpendicular recording media and perpendicular write heads are disclosed in U.S. Pat. No. 4,656,546 by Mallary and U.S. Pat. No. 4,748,525 by Perlov, which are incorporated herein by reference in their entirety.
The write head for perpendicular recording includes a write pole and a return pole that is coupled to the write pole. An electrically conductive magnetizing coil surrounds the yoke of the write pole to switch the polarity of the magnetic field applied to the write pole. During operation, the recording head flies above the magnetic recording medium by a distance referred to as the fly height. An electrical current is passed through the coil to create a magnetic flux within the write pole. The magnetic flux passes from the write pole tip through the magnetic recording layer and into a magnetically soft underlayer disposed beneath the magnetic recording layer. The soft magnetic underlayer causes the magnetic flux to pass across to the return pole. In addition, the soft underlayer produces magnetic charge images of the magnetic recording layer during read operations, increasing the magnetic flux and increasing the playback signal.
Perpendicular recording media can support higher areal densities than conventional longitudinal media, in part due to reduced demagnetizing fields in the recording transitions. While substantial advances have been made, further improvements in the magnetic recording media are necessary for the practical implementation of perpendicular magnetic recording devices. For example, the soft underlayer domains contribute to noise and lead to a lower signal-to-noise ratio (SNR). The magnetic domains in the soft underlayer also increase spike noises. Further, the remnant magnetization from the soft underlayer can lead to distortions in the giant magnetorestrictive (GMR) read sensitivity. Therefore, the soft underlayer should have high magnetization and permeability from the writing point of view and almost zero remnant magnetization from the reading point of view.
U.S. patent application Publication No. U.S. 2002/0028357 by Shukh et al. discloses a perpendicular magnetic recording medium with anti-ferro magnetic coupling in a soft magnetic underlayer. The soft magnetic underlayer includes a first magnetic soft layer, a first interface layer on the first magnetic soft layer, a second magnetic soft layer, a second interface layer on a second magnetic soft layer and a non-magnetic coupling layer between the first interface layer and the second interface layer.
U.S. patent application Publication No. U.S. 2003/0022023 by Carey et al. discloses a dual-layered type perpendicular magnetic recording disk for use in a perpendicular magnetic recording system. The magnetic recording disk includes a laminated underlayer that has at least two ferromagnetic films exchange-coupled across an anti-ferromagnetic coupling layer. The magnetic moments of the ferromagnetic layers in the laminated underlayer are oriented anti-parallel.
During the repeated application of magnetic fields from the head to write information on any particular track on the recording media, the leakage fields from the head can erase the information written on the adjacent tracks. This adjacent track erasure is considered to be a major hurdle to the implementation of perpendicular recording, particularly due to the magnification of head leakage fields by the magnetically soft underlayer that is necessary for perpendicular recording media.
There remains a need for an improved perpendicular recording medium having improved signal-to-noise ratio (SNR), a decreased pulse width (PW50) and reduced adjacent track erasure.